I've correctly stated the current state of the law; others are quoting conventional wisdom from before the most recent case on this:
"Failure to comply with the resulting court order would have resulted in contempt of court, enforceable by imprisonment." https://www.washingtonpost.com/opinions/be-warned-congress-subpoenas-may-not-uncover-the-trump-administrations-secrets/2018/11/29/8b5f39f8-f3ff-11e8-80d0-f7e1948d55f4_story.html …
1/ Essentially, Congress can go to the federal courts to get its subpoenas upheld. If the subpoena recipient doesn't comply with the court order to honor the subpoena, they can be imprisoned; if they do not show up to the hearing on the issue, they can be brought in (in custody).
2/ I used a shorthand in the first tweet of my thread that understandably confused people; that's my fault. The marshals get sent out by the court that's enforcing the Congressional subpoena, not Congress. It's a direct contempt situation, not a civil suit or "inherent contempt."
3/ Prior to the most recent litigation on this, others would have been correct that (incredibly) only a civil suit, DOJ charge, or Congress using its "inherent contempt" powers for the first time since 1930 would have allowed Congress to enforce a legally issued federal subpoena.
4/ I have no doubt that those who are Trump supporters or who simply, on principle, believe in a strong executive branch will say that the Supreme Court should overrule current precedent in the DC federal court. That's fine but has nothing to do with the situation Congress is in.
5/ I'll grant @rossgarber that what he calls a "civil suit" and I call a contempt proceeding with possible imprisonment involved (thus, arguendo, a "criminal" issue, though more properly a common law issue in equity, rather than law) may be seen as merely a semantic distinction.
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